Chicago may be the Windy City, but consider San Franisco, where gusts of 65 miles an hour have become common on downtown streets. The wind has become an official environmental concern there. Authorities say the reason is that skyscrapers, depending on their placement or design, can double or even triple the intensity of the wind at ground level. As a result, designers of new buildings 13 or more stories high must assess in advance their projects' effects on the wind. The city requires wind tunnel tests on models of high-rises being planned for construction. In one instance, city officials sent architects for a new bank building back to the drawing boards to redesign a rooftop park to minimize the wind's likely effects. The number of skyscrapers here is increasing steadily. In the past decade, 65 high-rise buildings -- with 30 million square feet of office space -- have been built or approved for the downtown area. "The whole face of downtown San Francisco is changing rapidly," says meteorologist Donald Ballanti, a consultant specializing in wind and air quality in the San Francisco Bay area. "Before, nobody thought much about the effects of big buildings on the wind. Now San Francisco has taken a strong stand to minimize wind effects." "Some of these buildings, depending on how they're designed, can produce lots of wind," notes Ballanti. "The wind comes in, hits the building and then swirls onto the street with extreme force." Strong winds hitting a cereal-box-shaped skyscraper at 9th and Market can quickly overturn newspaper racks, upset potted trees and even break store windows. The wide side of the building directly faces prevailing westerly winds, maximizing wind effects. Generally, Ballanti says, hte "best" buildings would have a narrow, tapered top and be placed on a platform, or "low pedestal," that would serve to divert the wind upwards, away from people and objects at street level. Conversely, he says, the "worst" buildings are plain, rectangular, slab-like structures that tend to catch a lot of wind and thrust it downward.